Modbus Exception: Data Out of Range (Holding Register Write)

What does 21 hex mean?

This Modbus exception occurs specifically when writing to a holding register if the data value is outside the parameter's permissible range. It's a more specific version of 'Illegal Data Value' (03 hex) for write operations to holding registers. The inverter rejects the write to prevent parameters from being set to an invalid state.

• Attempting to write a value exceeding the maximum or falling below the minimum allowed for a specific holding register (e.g., setting P1-01 (Accel Time) to 0.05s when min is 0.1s).
• The value being written requires specific units or scaling that is not correctly applied by the Modbus master (e.g., voltage in mV but sent in V).
• Parameter is dependent on other settings, and the new value, while seemingly valid, creates an invalid combination with other parameters.
• User input on the Modbus master (e.g., HMI) allows entering values beyond the inverter's limits without validation.
• Race condition or timing issue where another process changes a related parameter, making the attempted write invalid.

Click steps to track your progress.

1. 1

   1. Examine the inverter's manual for the specific parameter corresponding to the holding register, noting its minimum, maximum, and default values.

2. 2

   2. Review the Modbus master's programming logic to ensure the value being written is clamped or validated against the inverter's parameter limits.

3. 3

   3. Use a Modbus sniffer to verify the exact value transmitted by the master for the holding register in question.

4. 4

   4. Check if the parameter requires specific conditions (e.g., inverter stopped, motor type selected) before a particular value can be accepted.

5. 5

   5. Confirm that any scaling factors or unit conversions (e.g., 0.1s represented as 1 in the register) are correctly applied by the master.

6. 6

   6. Test writing boundary values (e.g., min-1, min, max, max+1) to the register to pinpoint the exact range violation.